1. Field of the Invention
This invention relates to gas-permeable membranes and their use in packaging, especially the packaging of fresh produce.
2. Introduction to the Invention
Fresh cut fruit and vegetables, and other respiring biological materials, consume oxygen (O2) and produce carbon dioxide (CO2), at rates which depend upon temperature and the stage of their development. Their storage stability depends on the relative and absolute concentrations of O2 and CO2 in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container whose permeability to O2 and CO2 is correlated with (i) the atmosphere outside the package, (ii) the rates at which the material consumes O2 and produces CO2, and (iii) the temperature, to produce an atmosphere within the container having O2 and CO2 concentrations equal to the optimum values for preservation of the material. The permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. No. 4,734,324 (Hill), U.S. Pat. No. 4,830,863 (Jones), U.S. Pat. No. 4,842,875 (Anderson), U.S. Pat. No. 4,879,078 (Antoon), U.S. Pat. No. 4,910,032 (Antoon), U.S. Pat. No. 4,923,703 (Antoon), U.S. Pat. No. 5,045,331 (Antoon), U.S. Pat. No. 5,160,768 (Antoon) and U.S. Pat. No. 5,254,354 (Stewart), and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosure of each of these publications is incorporated herein by reference.
The preferred packaging atmosphere depends on the stored material. For example, some materials, e.g. broccoli, are best stored in an atmosphere containing 1-2% O2 and 5-10% CO2. For other materials, an atmosphere containing 1-2% O2 and 12-30% CO2, e.g. about 15% CO2, is preferred. Thus, CO2 concentrations of 10 to 30% slow the respiration rate of some fruit and reduce the activity of some decay-causing organisms; for example, a CO2 concentration of 20% delays grey mold decay in rasberries and extends their shelf life.